you're right: on some keyboard configurations (like, for example, the "italian" one, wich i'm using...) you can't use the console immediately.
To access it open the file /Users/youruser/Library/Application Support/OpenArena/baseoa/q3config.cfg with a text editor (TextEdit will be fine), search for "toggleconsole" and change the "binded" key, then save.
For example i have
, so i can access to the console with the key "<". If you want to access to the console with the "c" key you'll have
and so on...
also, remember that on os x (i'm not sure on windows and linux...) you need to be in a map to open the console (you can't open it while in the menu. or, if you can, i don't know how to do it).